The present invention relates to a cutting insert, preferably an indexable cutting insert for turning, which insert comprises a micro chip breaker. The cutting insert may be single- or double-sided.
It is previously known to provide turning cutting inserts with chip breakers formed in the rake surfaces of the insert, which breakers extend along the edges of the insert which form cutting edges. These chip breakers may be formed in one or several steps.
Thus, in Swedish Patent 349 759 and Lundgren U.S. Pat. No. 3,792,515 a turning cutting insert is disclosed with two chip breaking steps being located behind each other, both of which are substantially parallel with the cutting edge. At smaller chip thicknesses, the chip is broken by the first chip breaking step, located next to the cutting edge. At increasing chip thickness, the chip passes to a larger extent over the first step and goes downwardly into the second chip breaking step. For tough materials that obtain a small initial bending, the chip will in some cases get downwardly the whole way to the bottom of the second step and be broken against it. This construction with two chip breaking steps gives a smoother chip breaking at larger chip thicknesses compared to previously used embodiments. However, at small feeds no satisfactory chip breaking is effected. A rule of thumb is usually that the smallest possible feed generally corresponds to the width of the primary land surface. The commercial cutting inserts corresponding to Swedish Patent 349 759 have a smallest primary land surface width of about 0.3 mm. Thus, if one uses smaller feeds than 0.3 mm, no acceptable chip breaking is attained with these cutting inserts.
When requirements of a smooth surface on the workpiece are high, fine turning is applied. This implies feeds down to 0.1 mm, which is not feasible in combination with good chip breaking with the known inserts referred to above.
A further disadvantage at small feeds is that the wear on the primary land increases, due to the direct abutment of the chips against this surface. Further, the flaking tendency of the surface coating increases when the inserts are coated, which is the most common case. These coatings usually consist of Al.sub.2 O.sub.3, titanium nitride and/or titanium carbide and are applied in a way known per se by so called PVD or CVD technique.
A primary object of the present invention is thus to provide a cutting insert which makes possible fine turning with an acceptable chip breaking.
A second object of the present invention is to increase the face wear resistance on the insert's primary land.
Still another object of the present invention is to improve the flaking resistance for surface coated inserts.
Yet another object of the present invention is to reduce the cutting forces.
A still further object of the present invention is to provide a cutting insert which makes possible both normal and fine machining.